<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style type="text/css">
			.demo {
				width: 300px;
				background: green;
				padding: 3px 0;
			}

			.box {
				/* width: 100%; */
				width: -moz-calc(100% - (10px + 5px) * 2);
				width: -webkit-calc(100% - (10px + 5px) * 2);
				width: calc(100% - (10px + 5px) * 2);
				background: yellow;
				height: 50px;
				padding: 10px;
				border: 5px solid green;
			}
		</style>
	</head>
	<body>
		<div class="demo">
			<div class="box"></div>
		</div>
	</body>
</html>
